It is becoming clear in these years that poly(oxazoline) (hereafter may be abbreviated as POx) are useful materials in the art of surface chemistry and biological materials, because they act as nonionic surfactant, protein modifier, hydrogel and carrier of medicines. Cationic ring-opening polymerization of oxazoline under adequate conditions is known to progress by living polymerization process to provide poly(N-acrylethyleneimine). A wide variety of POx can be produced by changing alkyl substituent or terminal group of starting oxazoline. Those POx's having short chain alkyl (e.g., methyl or ethyl group) at 2-position of side chain are water-soluble. Hydrophilicity of POx, however, decreases with increase in length of the alkyl substituent, until it becomes water-insoluble at all temperatures or at a certain fixed temperature. Of those POx's poly(2-isopropyl-2-oxazoline) (which hereafter may be abbreviated as PiPrOx) having isopropylcarbonyl group at 2-position of side chain are of particular interest. These polymers are soluble in cold water, and their aqueous solutions have their cloud points in the vicinity of physiological conditions (cf. Patent Reference 1 or Non-patent Reference 1 identified below. All References cited in this clause are collectively listed later). This is a property analogous to that of poly(N-isopropylacrylamide) which is a typical temperature-responsive polymer having versatile utilities.
Main merit of PiPrOx which are POx homologs is that they can be strongly expected to be biocompatible temperature-responsive polymers and hence are per se very useful in biomedical utilities. For example, liposomes modified with poly(2-ethyl-2-oxazoline) exhibit high biocompatibility and long blood circulation time (see Non-patent Reference 3) comparable to those of ordinary poly(ethylene glycol) lipopolymer (e.g., see Non-patent Reference 2). Besides, as temperature-responsive PiPrOx which are expected to open up new field of utility, monodispersible heterotelechelic PiPrOx having different functional groups at α-terminal and ω-terminal and the cloud point at about 37° C. have also been provided (cf. Non-patent Reference 4).    (1) Patent Reference 1: JP Hei 5 (1993)-310929A    (2) Non-patent Reference 1: Uyama, H., et al., Chem. Lett., 1992, 1643    (3) Non-patent Reference 2: Kataoka, K., et al., J. Controlled Release, 1993, 24, 119    (4) Non-patent Reference 3: Woodle, I. M., et al., Bioconjugate Chem., 1994, 5, 493    (5) Non-patent Reference 4: Park, J., et al., Macromolecules, 2004, 37, 6786